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If you have the volume of the speakers up, on Neo Geo logo you can hear a buzz and right after it where it says caution instructions for 1 sec the buzz changes to higher frequensy buzz for this sec, then it becomes the initial buzz sound.
I have put 4 104 caps and a 470uf on the sound chip. It didn't work.
But the 4 104 caps I used were larger in size (double size) compared to the caps MKL used. Maybe they were "Mylars" like Xian said. The shop had only them to give me.
EDIT Are mylars not suitable?

Hi guys, I have a problem with my japanese 3-6 AES. It has a RGB bypass mod and stereo mod getting the signal from the Headphones. Unfortunately I am getting a buzz sound on my Hifi no matter how I connect it. I had another Neo Geo with a stereo mod before which did not have the problem.

I used good shielded cables and the mod is correctly done. Is there any solution?

Hi guys, I have a problem with my japanese 3-6 AES. It has a RGB bypass mod and stereo mod getting the signal from the Headphones. Unfortunately I am getting a buzz sound on my Hifi no matter how I connect it. I had another Neo Geo with a stereo mod before which did not have the problem.

I used good shielded cables and the mod is correctly done. Is there any solution?

The audio signal is likely to be missing ground. Just connect it from the neo geo to the RCA cables (or whatever you're using), you can get it from a lot of places.

If you've already checked with a multimeter if there's continuity between pin 4, RCA Ground and a ground point on the board, there's definitely no ground issues and we can move forward.
Although buzzing sounds are 99% of times related to a missing ground connection.

One more thing: do you have all the power supplies plugged into the same power strip?

I remember once I was getting crazy because I had a loud buzz in the sound and couldn't understand where that came from. Ultimately I discovered I had a ground loop and solved by plugging all the power supplies in the chain to the same power strip.

A few years ago, after buying a system from me, the customer (who considered himself an audiophile) complained that he could hear a buzzing noise coming from his TV speakers as well as from the headphone jack. He also mentioned that he had a low serial system that didn't have that issue at all.

Since I had another (unmodded) system with the same PCB revision (3-4), I tested it immediately and much to my surprise it was having the same issues described by the guy. I couldn't believe I had never noticed it because once it was pointed out to me it was impossible not to hear it.

To get a better grasp of the problem I ran tests with as basic a setup as possible: I ran the system without hooking up video, only with headphones plugged in and another system (revision 3-6) running next to it, also with only headphones in and only the 3-4 was having the buzzing issue. Both system were powered by the same power supply (without step-down transformer) so the PSU was ruled out as a possible cause.

When the guy sent the system back to me along with his other "silent" system (that turned out to be the very first PCB type NEO-AES) the tests confirmed that 3-4 had the issue and that NEO-AES was buzz-free just like 3-6. The thing was all the more puzzling as NEO-AES and 3-4, despite being different revisions, have the same audio circuit (while on revisions 3-5 and 3-6 it's partly different).

I was starting to randomly put the blame on the different PCB layout of the 3-4 causing interference when I noticed the difference between the NEO-AES and 3-4 circuits: each audio IC (DAC, op-amps, headphone amplifier) on the NEO-AES has a decoupling (bypass) cap nearby (close to the VCC and GND leads) just like all the other ICs on the board which is what you would normally expect to see:

The 3-4 system doesn't have them though. Not a single bypass cap in sight:

After adding 0.1uF (104 marking) caps across the 5V and ground leads of each IC the issue was gone:

The cap for the DAC (YM3016) seems to make the most difference.

According to the datasheet, the bypass cap recommended for the headphone amplifier IC is a 100uF electrolytic cap. The value for this cap on MVS boards is 470uF. I put a 100uF (470uF is too bulky) in addition to the 0.1uF:

Of course you will have noticed the 1000pF (marking 102K) caps soldered to pin 1 and 16 of the IC. Those are other components that SNK left out of the headphone circuit on AES boards but they're supposed to be there according to the datasheet (they are part of a low-pass filter on the inputs) and are not missing from MVS boards:

A further example of the lack of attention in designing the audio circuit on this revision is the incorrect placement of two polyester caps in the L channel path:

To rectify things desolder and swap them.

How about the other revisions? Will 3-3 have the same issue as 3-4? I didn't have the chance to test one but judging by the lack of bypass caps for the audio ICs it's safe to assume it has the issue too:

I mentioned above that revision 3-6 is buzz-free. Let's take a look at the audio circuit of this board:

In this case the caps used for decoupling purposes are electrolytic instead of the usual multilayer ceramic ones. They're the 10uF caps numbered 1, 3 and 9 in the previous pics.

Revision 3-5 is almost identical to 3-6 but guess what, the bypass cap for the DAC (number 1 on 3-6) is missing:

I haven't tested a 3-5 system so I can't tell if the missing cap makes any difference.

It may be worth mentioning here that the headphone out on revision 3-6 has the R and L channels reversed. I had noticed this years ago and wasn't the only one:

Of course you will have noticed the 1000pF (marking 102K) caps soldered to pin 1 and 16 of the IC. Those are other components that SNK left out of the headphone circuit on AES boards but they're supposed to be there according to the datasheet (they are part of a low-pass filter on the inputs) and are not missing from MVS boards:

A further example of the lack of attention in designing the audio circuit on this revision is the incorrect placement of two polyester caps in the L channel path:

To rectify things desolder and swap them.

How about the other revisions? Will 3-3 have the same issue as 3-4? I didn't have the chance to test one but judging by the lack of bypass caps for the audio ICs it's safe to assume it has the issue too:

I mentioned above that revision 3-6 is buzz-free. Let's take a look at the audio circuit of this board:

In this case the caps used for decoupling purposes are electrolytic instead of the usual multilayer ceramic ones. They're the 10uF caps numbered 1, 3 and 9 in the previous pics.

Revision 3-5 is almost identical to 3-6 but guess what, the bypass cap for the DAC (number 1 on 3-6) is missing:

I haven't tested a 3-5 system so I can't tell if the missing cap makes any difference.

It may be worth mentioning here that the headphone out on revision 3-6 has the R and L channels reversed. I had noticed this years ago and wasn't the only one: